The inv(16) is one of the most frequent chromosomal translocations associated with acute myeloid leukemia (AML). This translocation fuses the promoter and most of the gene encoding the enhancer core binding factor-6 (CBFB) to MYH11, which encodes a smooth muscle myosin heavy chain to create the inv(16) fusion grotein (the "IFF"). CBFB acts as a co-factor for the RUNX1 transcription factor and the IFF stimulates RUNX1-dependent transcriptional repression. We found that the IFF contains a C-terminal repression domain that associates with the mSinSA co-repressor and histone deacetylase 8 (HDAC8) and that it cooperates with RUNX1 to repress the transcription of genes such as the pi4ARF tumor suppressor. Given that we also demonstrated that RUNX1 recruits mSin3A and HDACs, we hypothesize that the IFF traps RUNX1 in a complex with co-repressors and HDACs to create a dominant repressor of RUNX1-regulated genes. To test this hypothesis, we have developed a mouse model of inv(16)-induced AML that uses recombinant retroviruses to express the IFF in hematopoietic stem cells. While murine hematopoiesis is somewhat different that human hematopoiesis, the leukemia that the IFF induces in mice contains many of the hallmarks of the human inv(16)-related myelomonocytic AML. Importantly, the IFF repression domain, which contains the mSinSA and HDACS binding sites, is required for the in vivo action of the inv(16) in leukemogenesis. Because this mouse model is ideal for structure/function analyses, a major goal of this proposal is to identify the functional domains of the IFF that are required for leukemogenesis in vivo. Also, because the repression domain is a key functional domain, we will further dissect this domain to define the sequences that contribute to transcriptional repression and myeloid cell transformation. This includes determining the 3 dimensional structure of the minimal transcriptional repression domain of the IFF. Finally, we will use this mouse model to address fundamental questions as to how this chromosomal translocation causes acute leukemia, including whether continued expression of the IFF is required to maintain the leukemic phenotype. This information is critical for the future development of therapeutic approaches that target the IFF. By combining structural biology, biochemistry, and mouse a model, we anticipate collaborative synergy and expect rapid progress that may be quickly translated into novel therapeutic approaches.